Keep in mind equipment design and load calculations when choosing a heating or cooling system.

(The following is taken from the Sheet Metal and Air Conditioning Contractors' National Association's Residential Comfort System Installation Standards Manual.)

All heating or air-conditioning equipment should be selected on the basis of its Btuh (British thermal unit per hour) output, air-handling capacity, pumping capacity and thermal-transfer capability to handle the calculated design-heating or air-conditioning load. Energy-conservation legislation has led to the selection equipment sizes that are adequate for the tasks, but not oversized. All equipment must meet the requirements of ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 90.2, "Energy-Efficient Design of New Low-Rise Residential Structures," and other local codes.

The following sections provide good design-practice sizing information as an initial starting point in selecting the HVAC equipment. It is strongly recommended, however, that all manufacturer sizing and installation procedures should be followed and all applicable codes researched before making a final equipment selection.

Heat-producing equipment

Heat-producing equipment is designed and tested to generate and transfer a specific amount of heat-to-air or water at certain operating conditions. The equipment must produce a temperature rise in the air (water) that is within the approved range stated on the rating plate of the unit or in the manufacturer's published information.

A. External static-pressure capabilities of the unit must be sufficient to overcome the resistance of the supply and return duct systems, supply outlets and return intakes, the wet cooling coil and external air-cleaning devices.

B. Output capacity of heating equipment shall not be less than the calculated design-heat loss of the structure, and depending upon local code requirements, should not greatly exceed the calculated heat loss except as required to satisfy the manufacturer's next nominal size of unit.

Cooling equipment

Cooling equipment output capacity should be matched to calculated design heat gain, except as required to meet a manufacturer's next larger nominal size. Capacities of cooling equipment may be noted in number of cooling tons (1 cooling ton = 12 Btuh) or in Btuh at ARI (Air Conditioning and Refrigeration Institute) standard-rated conditions.

It may be better to select a slightly undersized cooling unit than a unit with excess capacity. An undersized unit not only has a lower initial cost and lower operating costs, but it will also provide better comfort conditions within the structure, as discussed earlier. An oversized unit will tend to cycle more frequently and may not be able to maintain uniform humidity within the structure.

Heating-equipment sizing

Properly matching equipment capacity to the design heat loss is essential. The heating capacity of air-source heat pumps is usually supplemented by auxiliary heaters, most often of the electric-resistance type. However, fossil fuel furnaces are used in certain applications.

Insufficient capacity of heating equipment results in an inability to hold indoor design temperatures during periods of low outdoor temperatures. Grossly oversized equipment can cause discomfort due to short on times and wide indoor-temperature swings. Gross oversizing will also contribute to higher energy usage, due to increased starting thermal transient losses, stopping thermal transient losses, off-cycle losses, and shortened motor life.

Furnaces

The output capacity of furnaces (in Btuh) should be listed on the equipment nameplate. All furnace nameplates will show evidence that the equipment has been listed or approved by a recognized testing laboratory, such as UL or AGA (Underwriters Laboratories or American Gas Association).

A. Furnaces must be fired only with the fuel for which they have been approved. Natural gas, liquid petroleum gas and fuel oil shall be supplied to the furnace in the volume and at the pressure required by the manufacturer's instructions.

B. Electric-resistance furnaces shall be Btuh output and shall be sized within three kilowatts of the calculated design requirements. The nameplate will indicate the minimum electric service required and show the electrical rating in volts, amperes or watts of each field-replaceable electrical component.

C. Furnaces must be a product of a manufacturer that has published data sheets indicating the equipment rating. When a furnace is used as an auxiliary heater for a solar-heating system, the furnace shall be sized to furnish 100 percent of the calculated heat loss of the structure.

When the installed refrigeration-cycle heating capacity is less than 100 percent of the calculated requirement, the auxiliary-plus refrigeration-cycle capacity shall not exceed 115 percent of the calculated design heating requirements. The total of the installed emergency heat and auxiliary heat should not exceed 80 percent of the design requirements.

(For information on ordering SMACNA's Residential Comfort System Installation Standards Manual, write SMACNA, 4201 Lafayette Center Drive, Chantilly, VA 20194 or call (703) 803-2989. See www.smacna.org/bookstore on the Internet.)